1. Field of the Invention
This invention relates to an observing optical system and an imaging optical system and, in particular, to an optical system used in an image display which can be held to an observer""s head or face or added to a cellular phone or a personal digital assistant or in an imaging apparatus such as a camera.
2. Description of Related Art
In recent years, the development of image displays, notably head- or face-mounted image displays which are intended to enjoy a large-sized picture by oneself has been promoted. Moreover, in keeping with the popularization of cellular phones and personal digital assistants, there is a heavy demand from observers who wish to view the images and data of the cellular phones and personal digital assistants through large-sized pictures.
Techniques that an observer is capable of viewing an image on the display surface of an image display means by using an optical system constructed with three surfaces including rotational asymmetrical transmitting and reflecting surfaces to display a virtual image are disclosed in Japanese Patent Preliminary Publication Nos. Hei 8-234137, Hei 9-197336, and Hei 9-197337. U.S. Pat. No. 5,959,781 discloses a technique that an observer is capable of viewing an image on the display surface of an image display means by using a totally reflecting surface and a beam splitter surface to introduce light into a concave mirror so that a magnified virtual image is displayed.
Each of Hei 8-234137, Hei 9-197336, and Hei 9-197337, however, has no means for correcting chromatic aberration because a prism used is made with a single medium.
Thus, chromatic aberration of magnification is under-corrected and it is difficult to obtain high resolution. In each of Hei 9-197336 and Hei 9-197337, the prism is configured to lengthen in a longitudinal direction (Y direction). In the case of U.S. Pat. No. 5,959,781, since the number of parts is large, cost is increased and an assembly is difficult. Furthermore, the problem arises that since a polarization beam splitter is used as a beam splitter to utilize polarization characteristics, a material with very low birefringence, such as glass, must be used as a prism medium and the prism weighs heavy. If the optical system is constructed of plastic material in order to avoid this weight problem, the polarization beam splitter cannot be utilized as the beam splitter and, for example, a half mirror must be used. This construction, however, causes the problem that light from an electronic image is weakened by the surface of the half mirror before the light reaches an observer""s eye and hence only a dark image can be observed.
It is, therefore, an object of the present invention to provide an observing optical system or an imaging optical system which is small in size, lightweight, and bright to such an extent that it can be used in a cellular phone, a personal digital assistant, or a head mounted virtual-image observing apparatus, and which has high resolution.
The optical system according to the present invention is interposed between a pupil plane and an image plane, and includes, at least, a first prism, a second prism, and a holographic element which is sandwiched between the first prism and the second prism and is cemented to these prisms. When a light beam traveling along the optical path connecting the pupil plane and the image plane through the optical system is called a first beam, the first prism has a 1-1 surface placed on the pupil side, combining the function of transmission with the function of reflection of the first beam; a 1-2 surface placed on the opposite side of the pupil with respect to the 1-1 surface; and a 1-3 surface placed on the image side, transmitting the first beam. The second prism includes a 2-1 surface having the same shape as the 1-2 surface and placed opposite thereto, transmitting the first beam at least twice, and a 2-2 surface placed on the opposite side of the pupil with respect to the 2-1 surface of the second prism, reflecting the first beam. The holographic element is designed so that the first beam, when entering the holographic element at a first incident angle, is diffracted and reflected and when entering the holographic element at a second or third incident angle, is transmitted. By doing so, the 1-2 surface of the first prism is designed to combine the function of transmission with the function of reflection of the first beam, and the 1-1 surface of the first prism is designed to be a totally reflecting surface such that when the first beam is incident on the 1-1 surface at an incident angle over a totally reflecting critical angle, it is reflected, while when the first beam is incident at an incident angle below the critical angle, it is transmitted. Whereby, the 1-1 surface combines the function of reflection with the function of transmission.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment when taken in conjunction with the accompanying drawings.